The present invention is directed to a reproducible process for uniformly etching silicon from a series of micromechanical structures used in electrical devices and the like, and more particularly to a process for forming and etching porous silicon from micromechanical structures. The Government has rights in this invention pursuant to Contract No. DE-AC04-76DP00789 awarded by the U.S. Department of Energy to AT&T Technologies, Inc.
Silicon is known to have exceptional mechanical, thermal and electrical properties. The use of single crystal silicon in fabricating a great diversity of micromechanical devices, such as pressure and vibration sensors and accelerometers, is known by utilizing the silicon patterning and etching technologies. Many of these devices require structures that often have complex and elaborate geometry. In order to make structures of complex geometry, the silicon must be patterned not only in the plane of the surface, but also in a vertical dimension. The patterning in the plane of the surface of a wafer, for example, can be easily done by conventional photolithography and etching processes.
Conventional silicon removal technology includes chemical etching of silicon in hydroxide baths at elevated temperatures. However, precise control of the amount of silicon to be removed is difficult with chemical etching techniques due to the strong etch rate dependence on the temperature and bath composition. Moreover, chemical etching tends to result in an etched area with a matte rather than the preferred mirror finish. Furthermore, chemical etching results in etches along crystallographic directions.
Another known etching process uses anodic dissolution of silicon in hydrofluoric acid. At high anodic biases, the silicon is known to electropolish. Although electropolishing technique would provide beneficial results, the process is nevertheless deficient and requires removal of all the silicon in the electrochemical step.
Accordingly, there remains a need in the art for a silicon etching technique that does not suffer from the disadvantages associated with the conventional etching methods.